Aagathan is the love story of Gautham and Shreya that turns sour because of Shreya's father, Major General Hareendra Natha Varma's one deed that comes back to haunt his family.

Seventy-year-old Sethumadhavan makes feeble attempts to experience the innocence that existed in his past.

Three girl students are found to have committed suicide in a boarding school in Alwaye under mysterious circumstances. Their boyfriends are interrogated by the cops, and one of them, Ananthu goes into depression. He happens to be the boyfriend of one of the dead girls, Gowri. The case remains unresolved. A year later, investigating officer Sreekanth's niece Parvati joins the school , and is given the same room where Gowri used to stay. Soon Parvati starts acting weird, which is noted by her classmates, and even the school driver and peon start to also see Gowri appearing at night in the school corridors. Gowri's spirit soon appears before Parvati, seeking her help. At times, it also gets into Parvati, making her act possessed. It is then known that Gowri and her two friends were very happy go lucky characters in the school, and something happens during a picnic which changes their lives forever.